The present invention relates generally to the field of optical communications devices, and more particularly to lasers.
Photonic integrated circuits (PIC) provide an integrated technology platform increasingly used to form complex optical circuits. PIC technology allows multiple optical devices, both active and passive, to be integrated on a single substrate. For example, PICs may comprise integrated lasers, integrated receivers, waveguides, detectors, semiconductor optical amplifiers (SOA), gratings, and other active and passive semiconductor optical devices. Monolithic integration of active and passive devices in PICs provides an effective integrated technology platform for use in optical communications.
A particularly versatile PIC platform technology is the integrated twin waveguide (TG) structure. Twin waveguide combines active and passive waveguides in a vertical directional coupler geometry using evanescent field coupling. The TG structure requires only a single epitaxial growth step to produce a structure on which active and passive devices are layered and fabricated. That is, TG provides a platform technology by which a variety of PICS, each with different layouts and components, can be fabricated from is the same base wafer. Integrated components are defined by post-growth patterning, eliminating the need for epitaxial regrowth. Additionally, the active and passive components in a TG-based PIC can be separately optimized, with post-growth processing steps used to determine the location and type of devices on the PIC.
The conventional TG structure, however, suffers from the disadvantage that waveguide coupling is strongly dependent on device length, due to interaction between optical modes. For PIC devices such as lasers, the interaction between optical modes results in an inability to control the lasing threshold current and coupling to passive waveguides as a consequence of the sensitivity to variations in the device structure itself. The sensitivity to variations arises from the interaction between the different optical modes of propagation in the conventional TG structure. This interaction leads to constructive and destructive interference in the laser cavity, which affects the threshold current, modal gain, coupling efficiency and output coupling parameters of the device. The conventional TG structure suffers from unstable sensitivity of performance characteristics due to device length, even/odd mode interaction, and variations in the layered structure.
In U.S. patent application Ser. No. 09,337,785, filed on Jun. 22, 1999, entitled xe2x80x9cTwin Waveguide Based Design for Photonic Integrated Circuits,xe2x80x9d the contents of which are hereby incorporated by reference in their entirety, Applicants disclose a modified TG structure, referred to as an asymmetric twin waveguide (ATG) which addresses some of the performance problems of the conventional TG structure. The ATG structure significantly reduces modal interference by confining different modes of light to propagation in different waveguides. This is accomplished by designing each of the single mode waveguides that are comprised in the twin waveguide such that the mode of light that propagates in each of the two waveguides has a different effective index of refraction. The asymmetric waveguides may be laterally tapered to reduce coupling losses by resonant or adiabatic coupling of the optical energy between the first and second waveguide. The asymmetric waveguide design significantly reduces the interaction between optical modes and therefore represents a great improvement over traditional TG devices.
While the ATG promises to be a versatile platform, Applicants recognized a need to deliver the photonic devices often touted, but never realized by PIC technology. Accordingly, in U.S. patent application Ser. No. 09/717,851, filed on Nov. 21, 2000, entitled xe2x80x9cPhotonic Integrated Detector Having a Plurality of Asymmetric Waveguides,xe2x80x9d the contents of which are hereby incorporated by reference in their entirety, Applicants disclose a photo-detector device based on the asymmetric waveguide design. An embodiment of the photo-detector PIC comprises more than two asymmetric waveguides. The asymmetric waveguide photodetectors are highly responsive and operable at very high frequencies.
The development of the asymmetric waveguide photo-detector device shows great promise for asymmetric waveguide PICs. However, the need still exists for the development of other types of PIC devices. More specifically there is a need for improved laser devices such as electroabsorption-modulated lasers (EMLs). EMLs are often employed as transmitters in optical fiber communication systems. An EML device typically comprises a laser integrated with an electroabsorption modulator at the laser output. The laser may be, for example, a distributed-feedback (DFB) or a distributed Bragg-reflector (DBR) device, which is operated continuously to ensure stability of output power and wavelength. The electroabsorption (EA) modulator is optically coupled to the laser output and modulates the signal generated by the laser.
There are numerous difficulties associated with creating a monolithically integrated, high speed EML. First, the active regions of the laser and modulator typically require quantum wells that emit at different wavelengths. This design restraint is generally satisfied by designing the laser and modulator with different thicknesses and/or with different materials. Also, efficient optical coupling is required between the laser and the modulator, as well as, in the case of a DBR laser, between the laser""s active region and grating. Furthermore, effective electrical isolation between the active devices of the laser and modulator is required to avoid crosstalk between the two devices.
Existing methods of fabricating monolithic EML devices typically involve either multiple semiconductor regrowth steps to separately fabricate the laser and EA modulator, or a single growth on a specially prepared substrate wherein dielectric masks create different bandgaps in adjacent regions of the wafer. Either of these methods is complex and typically results in poor yield and therefore very high costs for finished product.
Accordingly, there is a need in the art for an improved EML that provides efficient coupling and effective isolation and which can be manufactured relatively inexpensively.
Briefly, the present invention meets these and other needs in the art.
According to a first aspect of the invention, a monolithic asymmetric twin waveguide based electroabsorption-modulated laser device is provided. The laser device comprises a first waveguide having a gain region, such as a multi-quantum well region, for amplifying primarily a first mode of light, and a second waveguide having a modulator formed therein for modulating a second mode of light propagating in the second waveguide and having a different effective index of refraction than the first mode of light. The first waveguide is positioned vertically on top of the second waveguide and has a lateral taper formed therein for moving light between the first waveguide and the second waveguide. According to this first aspect of the invention, the first mode of light is amplified in the first waveguide and transferred into the second waveguide by the lateral taper. At the end of the second waveguide, the light encounters the modulator, which causes a modulated optical signal to be emitted from the device.
According to another aspect of the invention, an electroabsorption modulated laser device comprising more than two vertically integrated asymmetric waveguides is disclosed. The laser device comprises a first waveguide having a gain region, such as a multi-quantum well region, for amplifying primarily a first mode of light, a second waveguide having a distributed Bragg reflector therein and for guiding primarily a second mode of light having a different index of refraction from the first mode of light, and a third waveguide having a modulator formed therein for modulating a third mode of light propagating in the third waveguide and having a different effective index of refraction than the second mode of light. The first waveguide is positioned vertically on top of the second waveguide and the second waveguide is positioned vertically on top of the third waveguide. The first waveguide has a lateral taper formed therein for transferring light between the first waveguide and the second waveguide, and the second waveguide has a lateral taper formed therein for transferring light from the second waveguide to the third waveguide. The first mode of light is amplified in the first waveguide and transferred into the second waveguide by the lateral taper. The light propagates in the second waveguide as the second mode of light and is transferred into the third waveguide by the lateral taper. The modulator in the third waveguide causes the signal to be modulated at the output of the device.
According to yet another aspect of the invention, an electroabsorption modulated laser device employing a distributed feedback (DFB) laser is disclosed. The DFB modulated laser device comprises a first waveguide and a second waveguide. The first waveguide has a gain region and a grating therein to form a DFB laser. A signal out of the DFB laser is transferred via a lateral taper into the second waveguide wherein the signal is modulated. Light propagating in the first waveguide has a different effective index of refraction than the mode of light propagating in the second waveguide.
Modulated lasers in accordance with the invention provide efficient optical coupling between the laser and modulator as well as effective electrical isolation between the laser and modulator devices. Furthermore, modulated lasers in accordance with the invention can be manufactured through a process that requires only a single epitaxial growth step. Post-growth processing steps determine the location of the laser and modulator. This simplifies the manufacturing process and allows for a high yield, relatively low cost integration method.
Additional aspects of the invention are described in detail below.